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Abstract

We demonstrate the switching of a silicon nitride micro ring resonator (MRR) by using digital microfluidics (DMF). Our platform allows driving micro-droplets on-chip, providing control over the effective refractive index at the vicinity of the resonator and thus facilitating the manipulation of the transmission spectrum of the MRR. The device is fabricated using a process that is compatible with high-throughput silicon fabrication techniques with buried highly doped silicon electrodes. This platform can be extended towards controlling arrays of micro optical devices using minute amounts of liquid droplets. Such an integration of DMF and optical resonators on chip can be used in variety of applications, ranging from biosensing and kinetics to tunable filtering on chip.

Figures (7)

Frame excerpts from Media 1. A schematic diagram showing the concept of microring resonator switching using digital microfluidics. Left – the droplet does not interact with the microring resonator. Right – voltage is applied; the droplet covers the microring resonator and modifies its properties.

(a) A micrograph of a complete device showing the electrodes layout. (b) Zoom in on the area denoted by the red lines. The waveguides, microrings and central electrodes can be clearly observed. (c) SEM cross section view of the electrode, showing the silicon and the thermal oxide layer. (d) SEM top view on the silicon-nitride MRR before being covered by Cytop layer.

Experimental setup (top plate removed for visualization purposes). The lensed fibers are butt coupled to the waveguides. The DMF pads are wire bonded to the carrier pads to facilitate the connection to an external voltage source.

Optical characterization results of our device. (a) Transmission versus wavelength with the droplet covers the MRR (blue) and with the droplet shifted away of the MRR (black). (b). Cross section geometry of the simulated structure. The refractive index of each layer is given. The Cytop profile was verified by AFM and the waveguide core profile was verified by SEM. (c) Optical mode profile calculated by finite element method. The effective refractive indices with air/water cladding are shown above; indicate an effective index difference of 0.033. (d) Waveguide transmission vs. time, showing a ~1 millisecond time response.

Frame excerpts from Media 3. The media shows the real time operation of the device. The droplet moves into/out the MRR top, bringing the MRR to/from resonance, which can be clearly seen at the media. (a) The droplet does not cover the MRR, resonance condition is obtained. The output signal is weak. Light scattering from the MRR can be observed. (b) The droplet moves and cover the MRR which is now not in resonance. As a result, stronger output signal is observed. Light scattering from the MRR can be no longer observed.